In the technology of process measurements and automation, physical parameters, such as e.g. mass flow rate, density and/or viscosity, of a medium flowing in a pipeline are often measured using inline measuring devices, which include a vibratory measurement transducer, through which the medium flows, and a measurement and operating circuit connected thereto, for effecting reaction forces in the medium, such as e.g. Coriolis forces corresponding to the mass flow rate, inertial forces corresponding to the density of the medium and/or frictional forces corresponding to the viscosity of the medium, etc., and for producing, derived from these forces, measurement signals respectively representing mass flow rate, density and viscosity.
Such measurement transducers, especially those in the form of Coriolis mass flow meters or Coriolis mass flow/density meters, are described in detail e.g. in WO-A 04/038341, WO-A 03/076879, WO-A 03/027616, WO-A 03/021202, WO-A 01/33174, WO-A 00/57141, WO-A 98/07009, U.S. Pat. Nos. 6,711,958, 6,666,098, 6,308,580, 6,092,429, 5,796,011, 5,301,557, 4,876,898, EP-A 553 939, EP-A 1 001 254, EP-A 1 448 956, or EP-A 1 421 349. For conveying the medium, flowing at least at times, the measurement transducers include at least one measuring tube, which is secured appropriately oscillatably to a usually thicker-walled, especially tubular and/or beam-like, carrier cylinder or in a carrier frame. For producing the above-mentioned reaction forces, the measuring tube is caused to vibrate during operation, driven by a, usually, electrodynamic exciter mechanism. For detecting vibrations of the measuring tube, especially inlet, and outlet, end vibrations, and for producing at least one oscillation measurement signal representing such, these measurement transducers additionally include a sensor arrangement reacting to movements, and thus also to mechanical oscillations, of the measuring tube.
During operation, the above-described, inner oscillation system of the measurement transducer, formed by the at least one measuring tube, by the medium conveyed at least instantaneously therein, and, at least in part, by the exciter mechanism and sensor arrangement, is excited by means of the electromechanical exciter mechanism, at least at times, to execute mechanical oscillations in a wanted oscillation mode at at least one, dominating, wanted oscillation frequency. These oscillations in the so-called wanted oscillation mode are, mostly, especially in the case of application of the measurement transducer as a Coriolis mass flow and/or density meter, at least partially developed as lateral oscillations. In such case, the wanted oscillation frequency is selected to be a natural, instantaneous resonance frequency of the internal oscillation system, which, in turn, depends on size, form and material of the measuring tube, as well as also on the instantaneous density of the medium; where appropriate, the wanted oscillation frequency can also be significantly influenced by an instantaneous viscosity of the medium. Due to fluctuating density of the medium to be measured and/or due to medium changes effected during operation, the wanted oscillation frequency is naturally changeable during operation of the measurement transducer, at least within a calibrated, and, to that extent, predetermined, wanted frequency band, which has, correspondingly, predetermined lower and upper frequency limits.
The inner oscillation system of the measurement transducer formed together by the at least one measuring tube, the exciter mechanism and the sensor arrangement is, furthermore, usually accommodated in a transducer housing having, as an integral component, a carrier frame, or carrier cylinder, as the case may be. This housing can likewise have a large number of natural oscillation modes. Suitable transducer housings for vibration-type measurement transducers are described, for example, in WO-A 03/076879, WO-A 03/021202, WO-A 01/65213, WO-A 00/57141, U.S. Pat. Nos. 6,776,052, 6,711,958, 6,044,715, 5,301,557 or EP-A 1 001 254. The housing caps of such transducer housings are usually manufactured in one piece by means of deep-drawn intermediates. Additionally, however, these housing caps can, especially in the case of larger dimensions, be composed of separate, shell-shaped, intermediate pieces, as is proposed e.g. also in WO-A 03/021202. The transducer housing described in WO-A 03/021202 is formed by means of a support tube and a housing cap welded therewith, with the housing cap itself including, due to the special manufacturing, an upper, essentially trough-shaped, first housing segment with a first segment edge and a second segment edge formed essentially identically to the first segment edge, an essentially planar, second housing segment, which is connected via its first segment edge with the first segment edge of the first housing segment, and a third housing segment essentially mirror-symmetric to the second housing segment and connected via its first segment edge with the second segment edge of the first housing segment.
Transducer housings of the described kind serve, besides for holding the at least one measuring tube, additionally also for protecting the measuring tube, the exciter mechanism and the sensor arrangement, as well as other internally situated components, from external, environmental influences, such as e.g. dust or water spray. The user also frequently requires that such transducer housings, and especially their housing cap, be able to withstand, leak-free, at least for a predetermined time, the internal pressure, mostly lying markedly above the external pressure, arising in the case of a bursting of the tube segment of the measuring tube. At least for applications involving toxic or easily ignitable media, the transducer housing must, in cases, also be able to fulfill the requirements applicable for a safety container. Additionally, also a sufficient damping of sound emissions possibly produced by the measurement transducer is required.
Development in the field of vibration-type measurement transducers has, in the meantime, reached such a level, that modern measurement transducers of the described kind can be used practically for almost all applications of flow measurement technology and can satisfy the highest requirements of this field. Thus, such measurement transducers are used in practice for mass flow rates of only a few g/h (grams per hour) up to several t/h (tonnes per hour), at pressures of up to 100 bar for liquids or even over 300 bar for gases. The achieved accuracy of measurement in such cases lies usually at about 99.9% of the actual value, or higher, or a measurement error of about 0.1%, with a lower limit of the guaranteed measurement range lying quite easily at about 1% of the measurement range end value. Due to the high bandwidth of their possibilities for use, measurement transducers of the described kind are offered, depending on application, additionally with nominal diameters lying, measured at the flange, between 1 mm and 250 mm or even above.
As the nominal diameters of vibration-type transducers become always larger, their installed mass practically inherently also becomes larger. Such measurement transducers, including flanges possibly attached thereto, have, in the meantime, grown, at least in individual cases or small-series production, to installed masses of far above 500 kg. However, it must be appreciated that, alone in consideration of the structural situations in the plants, it is necessary that there be limitations to further marked increases in the installed mass of such measurement transducers. Considering also that the installed mass increases more than proportionally to the nominal diameter of the measurement transducer, in order to achieve the high mechanical stability likewise required for measurement transducers of the described kind, it seems that the above-mentioned sizes already represent an upper limit for what is currently economically realizable for vibration-type measurement transducers. In the case of the above-described, conventional forms of construction, a corresponding installed mass to nominal diameter ratio of the total installed mass of the measurement transducer to its nominal diameter, for nominal diameters of less than 150 mm, is usually smaller than 1 kg/mm, while, for nominal diameters of over 150 mm, especially greater than 200 mm, the ratio would lie noticeably above 1.5 kg/mm. Considering that, in the case of measurement transducers of the described form of construction with nominal diameters of greater than 150 mm and with use of the currently usual materials, very high installed mass to nominal diameter ratios are to be expected, it appears that, for vibration-type measurement transducers, an increase of their nominal diameters is scarcely possible any more, without accompanying significant increase of the installed masses.
As a result of the specified limitations respecting maximum installed masses, a special problem exists for the design of measurement transducers of large nominal diameter, that, due to the then compelled very high total mass of the above-mentioned inner oscillation system (mass of the measuring tube itself, mass of the volume fraction of the medium to be measured instantaneously conveyed in the measuring tube, total mass of the exciter mechanism and sensor arrangement, etc.), an outer oscillation system of the measurement transducer, formed at least by the transducer housing, including carrier cylinder, or carrier frame, as the case may be, and possibly provided distributer pieces and/or flanges, must, in comparison to the inner oscillation system, become ever lighter. In other words, such measurement transducers with large nominal diameters must, because of their most often large installed mass, be so designed, that, in comparison with conventional measurement transducers with smaller nominal diameters, a mass ratio of a total mass of the outer oscillation system to a total mass of the inner oscillation system is small.
Investigations have now, however, shown, that, in the case of comparatively small mass ratios (total mass of the outer oscillation system:total mass of the inner oscillation system) of smaller than 4:1, such as can arise due to the above-mentioned limiting to a still-manageable installed mass of the measurement transducer, especially in the case of measurement transducers of large nominal diameter, especially in the case conventionally constructed measurement transducers of a nominal diameter of greater than 200 mm, eigenfrequencies of the outer oscillation system unluckily become shifted quite near to the wanted oscillation frequency or even into the wanted frequency band. As a result of this, the undesired situation can, for example, arise, that the inner oscillation, operating, as it should, at the wanted oscillation frequency, excites the outer oscillation system to resonance oscillations, which then get superimposed on the oscillations of the inner oscillation system and, thus, can significantly influence, or even render completely useless, the oscillation measurement signal delivered by the sensor arrangement. The interfering vibrations are, in such case, caused to a considerable degree by the components of the outer oscillation system, especially the mentioned housing segments, which are made with a wall thickness mostly smaller than 5 mm, thus almost thin-walled, yet being at the same time quite large as regards surface area. For example, the frequency spectrum shown by way of example in FIG. 2 was experimentally determined for an outer oscillation system of form of construction described in WO-A 03/021202 and schematically illustrated in FIGS. 1a, b, including a support tube and a housing cap affixed thereto. Clearly recognizable is that the outer oscillation system exhibits pronounced oscillation modes at about 255 Hz and about 259 Hz, with the above-mentioned, wanted frequency band for the inner oscillation system of the same measurement transducer having been determined to lie in the range of about 210 Hz to 270 Hz. According to this, in the case of the described measurement transducer configuration, the outer oscillation system would resonate over practically the entire wanted frequency band that should actually be kept free of disturbances. Consequently, the oscillation measurement signals determined in such case, especially signals for a mass flow rate measurement or for a density measurement, would be essentially completely unusable.
A possibility for reducing such disturbance oscillations coming from the outer oscillation system is, for instance, as proposed e.g. in WO-A 01/33174, to affix extra masses to the transducer housing, such as resonate essentially with the transducer housing and, therefore, bring about a studied detuning of the outer oscillation system relative to the inner oscillation system. A disadvantage of such a solution is, with reference to the use on measurement transducers of large nominal diameter, that this, in turn, results in a further increasing of the already very large installed mass of the measurement transducer.